Fix deprecated copy warnings in amount.h

Summary: This is a warning that is enabled by default on modern versions of GCC. Out code is tripping this in the Amount class.

Test Plan: Build and ensure that warnings in Amount when building with GCC9.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3963